Regulation and expression of MHC class H genes are considered central in importance for understanding antigen presentation and T cell activation in the pathogenesis of multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE). Interferon-gamma (IFNgamma), a pleiotropic cytokine, is considered a requirement for class II induction on nonprofessional antigen presenting cells (APC), including resident CNS APC (microglia, astrocytes, and endothelial cells) thought to participate in antigen presentation and initial CD4+ T cell activation in CNS inflammation. Yet, mice deficient in INFgamma are susceptible to EAE. The MHC class 11 transactivator (CIITA) is a key intermediate in IFNgamma-inducible and constitutive class II expression. CIITA was mapped to human chromosome 16pl3, a region associated with MS susceptibility. CHTA expression is controlled in a tissue-specific manner by differential activation of multiple promoters, one that directs IFNgamma-inducible CIITA expression in non-professional APC, and two that direct constitutive CIITA expression in professional APC. The applicants hypothesize that CIITA is critical for class H expression by APC responsible for CNS antigen presentation, and that some APC can participate in initial CD4+ T cell activation in the CNS using an IFN-gamma-independent, CIITA-dependent pathway to direct class II expression. The applicants hypothesize that polymorphism(s) within the CIITA gene complex contribute to MS susceptibility. The specific aims of this proposal are: (1) To evaluate CIITA regulation and class H expression by APC in CNS inflammation through analysis of EAE in normal mice, IFNgamma-deficient mice, and IRF-1-deficient mice using immunohistochemistry, Rnase protection assays, and in situ hybridization. (2) To examine how constitutive CNS CIITA expression influences susceptibility to CNS inflammatory disease. (3) To determine whether polymorphisms associated with the CIITA gene complex contribute to MS susceptibility by sequencing of CIITA promoter elements and CIITA cDNA in a select number of patients, to analyze these polymorphisms in a familial MS dataset, and to narrow the candidate region using high resolution genotyping of l6pl3. These studies may provide valuable information regarding regulation of class II expression by APC in CNS inflammation, which may provide insight for development of therapeutic means to modulate class H expression and T cell activation in MS.